Multi-user MIMO systems and methods

ABSTRACT

A method and system are provided for scheduling data transmission in a Multiple-Input Multiple-Output (MIMO) system. The MIMO system may comprise at least one MIMO transmitter and at least one MIMO receiver. Feedback from one or more receivers may be used by a transmitter to improve quality, capacity, and scheduling in MIMO communication systems. The method may include generating or receiving information pertaining to a MIMO channel metric and information pertaining to a Channel Quality Indicator (CQI) in respect of a transmitted signal; and sending a next transmission to a receiver using a MIMO mode selected in accordance with the information pertaining to the MIMO channel metric, and an adaptive coding and modulation selected in accordance with the information pertaining to the CQI.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of Ser. No. 12/089,938, filed Apr.11, 2008, which is a Submission Under 35 U.S.C. §371 for U.S. NationalStage Patent Application of International Application Number:PCT/CA2006/001665, filed Oct. 12, 2006, entitled “MULTI-USER MIMOSYSTEMS AND METHODS,” which claims priority to U.S. ProvisionalApplication Ser. No. 60/725,951, filed Oct. 12, 2005, the entirety ofall of which are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

n/a

FIELD OF THE INVENTION

The invention relates to communication systems in general, andparticularly to MIMO (multiple-input multiple-output) communicationsystems.

BACKGROUND OF THE INVENTION

In a MIMO communication system, a transmitter transmits data throughmultiple transmitting antenna (N_(T)) and a receiver receives datathrough multiple receiving antenna (N_(R)). The binary data to betransmitted is usually divided between the transmitting antennae. Eachreceiving antenna receives data from all the transmitting antennae, soif there are M transmitting antennae and N receiving antennae, then thesignal will propagate over M×N channels, each of which has its ownchannel response.

MIMO wireless communication systems are advantageous in that they enablethe capacity of the wireless link between the transmitter and receiverto be improved compared with previous systems in the respect that higherdata rates can be obtained. The multipath rich environment enablesmultiple orthogonal channels to be generated between the transmitter andreceiver. Data can then be transmitted over the air in parallel overthose channels, simultaneously and using the same bandwidth.Consequently, higher spectral efficiencies are achieved than withnon-MIMO systems.

SUMMARY OF THE INVENTION

In some aspects of the present invention, a base station in a multi-userMIMO system selects a transmission method on the basis of feedbackinformation received from a plurality of receivers.

In some aspects, the base station assigns a data rate and a MIMO modesuited to the channel quality for that user.

In some aspects, the present invention includes systems and methodswhich may compute MIMO channel metrics.

In some aspects, the present invention includes systems and methodswhich may include MIMO mode selection.

In some aspects, the present invention includes systems and methodswhich may assign/schedule MIMO user transmission and associated formatsin order to maximize MIMO communication capacity.

In some aspects, the present invention includes systems and methodswhich may be used in conjunction with OFDM sub-channels.

In some aspects, the present invention includes systems and methodswhich use uplink

(UL) channel sounding where MIMO matrices may be calculated on thetransmission side.

According to one broad aspect of the present invention, there isprovided a method in Multiple-Input Multiple-Output (MIMO) transmitter,the method comprising: a) generating or receiving information pertainingto a MIMO channel metric and information pertaining to a Channel QualityIndicator (CQI) in respect of a transmitted signal; b) sending a nexttransmission to a receiver using a MIMO mode selected in accordance withthe information pertaining to the MIMO channel metric, and an adaptivecoding and modulation selected in accordance with the informationpertaining to the CQI.

According to another broad aspect of the present invention, there isprovided a method in a MIMO receiver comprising: a) generating a MIMOchannel metric and CQI of a received signal at the receiver; b) feedingback information pertaining to the MIMO channel metric and informationpertaining to the CQI.

According to still another broad aspect of the present invention, thereis provided a computer program product comprising a computer-readablemedium storing instructions which, when executed by a processor,generate information pertaining to a MIMO channel metric and informationpertaining to a CQI in respect of a transmitted signal; and instruct atransmitter to send a next transmission to a receiver using a MIMO modeselected in accordance with the information pertaining to the MIMOchannel metric, and an adaptive coding and modulation selected inaccordance with the CQI.

According to yet another broad aspect of the present invention, there isprovided a system for processing signals received from a receivercomprising: an input for receiving the signals; and a processorconfigured to: generate or receive information pertaining to a MIMOchannel metric and information pertaining to a CQI of the signals;select a MIMO mode based on the information pertaining to the MIMOchannel metric and the information pertaining to the CQI.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of the specific embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail with reference tothe accompanying diagrams, in which:

FIG. 1 is a block diagram of a multi-user MIMO system in accordance withan embodiment of the invention where channel measurements are calculatedon the receive side;

FIG. 2 is a block diagram of a multi-user MIMO system in accordance withan embodiment of the invention where channel measurements are calculatedon the transmission side;

FIG. 3A illustrates a lookup table for an embodiment of the invention inwhich a Forward Error Correction (FEC) code, a modulation type and aMIMO mode are selected based on a Channel Quality Indicator (CQI);

FIG. 3B illustrates a key table for FIGS. 3A and 4A;

FIG. 4A illustrates a lookup table for another embodiment of theinvention in which a Forward Error Correction (FEC) code and amodulation type are selected based on a CQI;

FIG. 4B illustrates a lookup table for a MIMO mode based on a MIMOindicator (MIMOI);

FIG. 5 is a diagram illustrating a graphical lookup for a MIMO modebased on a MIMOI and a CQI;

FIG. 6 is a plot of BLER (Block Error Rate) versus Signal-to-Noise Ratio(SNR) for an embodiment of the invention;

FIG. 7 is a plot of BLER (Block Error Rate) versus SNR for anotherembodiment of the invention;

FIG. 8A is a plot of Cumulative Distribution Function (CDF) versuscondition number of H^(H)H for an embodiment of the invention; and

FIG. 8B is a plot of CDH versus H^(H)H for another embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

According to embodiments of the present invention, systems and methodsare provided which enhance the performance of communication channels ina communication system, to thereby improve, for example, thetransmission performance of multi-user MIMO communication systems.

In multi-user MIMO systems, a multi-data stream transmitter at a basetransceiver station (BTS) that provides communication services for acoverage area or cell in a wireless communication system transmitscommunication signals to a plurality of user terminals via multipleantennas. User terminals are also commonly referred to as MIMOreceivers, user equipment (UE), communication devices, and mobilestations, for instance. At a MIMO receiver side, multiple receiveantennas are employed for each user.

FIG. 1 is a block diagram of a multi-user MIMO system in accordance withone embodiment of the invention. On the transmit side, the system ofFIG. 1 includes a BTS 100 with an Adaptive Coding and Modulation Module102, user feedback module 108, and a pair of antennas 110, 112. On thereceive side, the system of FIG. 1 includes one or more user terminals119, 125, 131 (three shown in the illustrated example) includerespective MIMO receivers 118, 124, 130 each having a pair of antennas114,116, 120,122, and 126,128 respectively.

In user terminal 131, antennas 126, 128 are both connected to the MIMOreceiver 130 and to a MIMO channel module 132. MIMO channel module 132represents the real world radio propagation channel. MIMO receiver 130is connected to MIMO channel metric measurement module 134. MIMO channelmodule 132 is connected to CQI metric measurement module 136. Both MIMOchannel metric measurement module 134 and CQI metric measurement module136 are connected to composite feedback module 138. Composite feedbackmodule 138 forms part of the feedback path from the receive side to thetransmit side. Information regarding the MIMO channel metric and the CQImetric is transmitted from MIMO channel metric measurement module 134and CQI metric measurement module 136 respectively to composite feedbackmodule 138 which incorporates one or more lookup tables to determine aMIMO mode and data rate. The MIMO mode and data rate are fed back bycomposite feedback module 138 to BTS 100 by any convenientcommunications method, which may or may not comprise wirelesscommunications.

Each of user terminal 119 and user terminal 125 also include channelmeasurement modules as well (i.e. each have their own modules equivalentto MIMO channel module 132, MIMO channel metric measurement module 134,CQI metric measurement module 136, and composite feedback module 138).These modules, which are connected to each of MIMO receiver 118 and MIMOreceiver 124, have been intentionally omitted to simplify FIG. 1.

The system of FIG. 1 operates as follows. Pilot data is input intoadaptive coding and modulation module 102 where such pilot data isconverted into communication signals which may then be transmitted viathe antennas 110,112 from the BTS 100 to user terminals 119, 125 and131. In some embodiments, the pilots are inserted on each antenna in amanner that makes them distinguishable at a receiver. For example, forOFDM implementations, a respective set of sub-carrier and OFDM symboldurations can be employed for each antenna.

At MIMO receivers 118, 124, and 130, each of the antennas 114,116,120,126, and 126,128 receive the pilot signals transmitted from theantennas 110,112. MIMO receiver 130 processes the received signals toproduce separated layer signals which are fed to MIMO channel metricmeasurement module 134. The MIMO channel measurement metric measurementmodule 134 processes the received pilot data having regard to knowledgeof what the transmitted pilot data was, and produces a MIMO channelmetric. Specific examples of calculations which may be performed toassesses a MIMO channel metric are described below. MIMO channel module132 processes the received signal to produce MIMO channel stateinformation which is fed to the CQI measurement module 136. The CQImetric measurement module processes the received pilot data havingregard to knowledge of what the transmitted pilot data was, produces aCQI metric. CQI metrics are well known and may for example include CINR(carrier to interference and noise ratio), and the rank of the MIMOchannel.

The CQI metric is used as a basis for selecting a particular coding andmodulation. BTS 100 can adjust the modulation order and/or coding ratein accordance with the CQI metric. More particularly, the datatransmission rate can be increased, decreased, maintained at a constantlevel, or reduced to 0 bits/s. In a particular example, the CQI is CINRas indicated above, and each range of CINR is associated with arespective adaptive coding and modulation.

In some embodiments, MIMO receivers 118, 124 and 130 track the channelquality via the pilot symbols received and accumulate these qualitymeasurements over a period of time to produce the CQI.

In some embodiments, the feedback from user terminals 119, 125, 131 mayalso include information identifying the receiver's MIMO capability. Forexample, this might indicate a number of receive antennas, or the rankof the MIMO channel.

The MIMO channel metric is used to select a MIMO transmission mode to beused for transmitting to a particular user terminal. The particular MIMOmodes that are available are selected on an implementation specificbasis. Four examples of MIMO modes include beamforming, BLAST,space-time transmit diversity (STTD), and spatial multiplex, though theinvention is in no way limited to these MIMO modes and is in factapplicable to all possible space-time mapping.

Those skilled in the art will appreciate that MIMO channel metricmeasurement and CQI metric measurement may be performed by a digitalsignal processor (DSP) or a general-purpose processor adapted to executesignal processing software, for example. Various techniques fordetermining such metric measurements will be apparent to those skilledin the art.

Both the MIMO channel metric and the CQI is transmitted to compositefeedback module 138 where one or more lookup tables may be used todetermine a composite metric used by BTS 100 to select a MIMO mode anddata rate. As used herein, “composite” can be equated to the “overall”quality of the channel matrix. The lookup carried out by compositefeedback module 138 is used for two purposes: (i) User terminal pairing,i.e. scheduling. The more orthogonal the channel, the larger the MIMOcapacity; and (ii) together with SNR, the lookup is used for MIMO modeand coding and modulation selection. With a higher SNR and compositemetric, spatial multiplexing and higher modulation and coding rates maybe selected. With a lower SNR and composite metric, transmit and lowermodulation and coding rates may be selected.

Note that the composite metric does not affect modulation and codingrates selection in transmit diversity, but it affects modulation andcoding rates selection in spatial multiplexing. This is because when thecomposite metric is low, more inter-layer interference will occur, andhence only lower modulation and coding rates are to be used.

The composite metric is then transmitted by composite feedback module138 to BTS 100 through user feedback module 108. With the compositemetric received from composite feedback module 138, a scheduler whichforms part of BTS 100 determines a MIMO transmission mode and amodulation and coding to be used for each MIMO receiver. In someembodiments, the BTS 100 indicates the transmission format to each MIMOreceiver.

In some embodiments, a two bit composite metric is used, with one bit ofthe composite metric being used to indicate the CQI, and one bit of thecomposite metric to indicate the MIMO mode, e.g. transmit diversity orspatial multiplexing. In the spatial multiplexing mode, one additionalbit can be used to indicate if the MIMO channel is orthogonal.

FIG. 2 is an alternative embodiment to that illustrated in FIG. 1. Inthis embodiment, channel metrics are measured at the transmit side 250rather than the receiver side 201. On the transmit side 250, the systemof FIG. 2 includes a BTS 200 with an Adaptive Coding and ModulationModule 202, user feedback module 208, and a pair of antennas 210,212. Onthe receive side 201, the system of FIG. 2 includes one or more userterminals 219, 225, 231 (three shown in the illustrated example) includerespective MIMO receivers 203, 204, 206 each having a pair of antennas214,216, 220,222, and 226,228 respectively.

In user terminals 219, 225, and 231, antennas 214,216, 220,222, and226,228 respectively are connected to MIMO receivers 203, 204, and 206which each perform UL channel sounding. In the case of Time DivisionDuplex (TDD), channel sounding is used to allow BTS 200 to performchannel measurements at the transmit side 250 rather than the receiverside 201. Information received from MIMO receivers 203, 204, and 206 isfed back through a feedback control channel to user feedback module 208at BTS 200 by any convenient communications method, which may or may notcomprise wireless communications.

User feedback module 208 is connected to both MIMO channel metricmeasurement module 234 and CQI metric measurement module 236. Both MIMOchannel metric measurement module 234 and CQI metric measurement module236 are connected to composite feedback module 238. Composite feedbackmodule 238 is connected to adaptive coding and modulation module 202.

Except for the fact that channel measurements are performed at thetransmit side 250 rather than the receive side 201, the operation of thesystem of FIG. 2 is otherwise similar to the operation of the system ofFIG. 1. The main difference is that through channel sounding, userterminals 219, 225, and 231 pass the burden of channel measurements andprocessing to BTS 200.

Of course, the systems of FIGS. 1 and 2 are only two illustrativeexamples of systems in which the invention may be implemented. Theinvention is in no way limited thereto. Extension of the principles ofthe present invention to systems having other dimensions will beapparent to those skilled in the art. In particular, the number of userterminals that will be present in a given implementation may differ, andmay vary over time if they are mobile. The number of antennas on thebase station and user terminals is two in the illustrated example. Moregenerally, any number, two or more, of antennas can be employed suchthat MIMO communications are possible, though the number of receiveantennas must be greater than to equal to the number of data streams(i.e. layers) being transmitted. Both the base station and userterminals include functionality not shown as would be understood to oneof skill in the art. Separate components are shown for each of the MIMOchannel 132,232, MIMO channel metric measurement module 134,234, the CQImetric measurement module 136,236, and the composite feedback module138,238. More generally, the functions provided by these modules may becombined in one or more functional elements, and these may beimplemented in one or a combination of software, hardware, firmware etc.

A MIMO system can be expressed as{right arrow over (y)}=H{right arrow over (s)}+{right arrow over (η)},where

{right arrow over (y)}=[y₁ y₂ . . . y_(N)]^(T) is a vector ofcommunication signals received at a receiver;

{right arrow over (s)}=[s₁ s₂ . . . s_(M)]^(T) is a vector ofcommunication signals transmitted by a transmitter;

{right arrow over (η)}=[η₁ η₂ . . . η_(N)]^(T) is a vector of noisecomponents affecting the transmitted communication signals;

$H = \begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1M} \\h_{21} & h_{22} & \ldots & h_{2M} \\\vdots & \vdots & \ddots & \vdots \\h_{N\; 1} & h_{N\; 2} & \ldots & h_{NM}\end{bmatrix}$is a channel matrix of communication channel attenuation factors;

N is a number of antennas at the receiver; and

M is a number of antennas at the transmitter.

For a [2Tx, 2Rx] MIMO channel,

$H = \begin{bmatrix}h_{11} & h_{12} \\h_{21} & h_{22}\end{bmatrix}$

A The eigenvalue of H^(H)H are λmax, λmin There are several schedulingapproaches, including orthogonality and capacity. These approaches arefor user terminal pairing only.

Where it is desired that scheduling by a BTS (such as BTS 100 and BTS200 in FIGS. 1 and 2 respectively) be provided on the basis of maximumorthogonality, the following MIMO channel metric will be computed by,for example, MIMO channel metric measurement module 134 in FIG. 1:

$\max\left\{ \frac{\det\left( {H^{H}H} \right)}{{trace}\left( {H^{H}H} \right)} \right\}$

The larger the metric, the more orthogonal is the channel.

For a maximum orthogonality decomposition scheduling scheme,

${\min{{\begin{bmatrix}h_{11} & h_{21}\end{bmatrix}\begin{bmatrix}* \\h_{12} \\* \\h_{22}\end{bmatrix}}}} = 0$

In this case, the channel is completely orthogonal, yielding twoseparate spatial channels, with channel attenuation factors being√{square root over (|h₁₁|²+|h₂₁|²)} and √{square root over(|h₁₂|²+|h₂₂|²)} respectively.

For scheduling based on a best conditional number of MIMO channelscheme, the following channel metric will be calculated by, for example,MIMO channel metric measurement module 134 in FIG. 1:ρ=λmax/λmin=˜1

In this case, an advanced receiver (maximum likelihood detection) and/ora simplified receiver can be employed.

For scheduling based on maximum capacity, the following metric will becalculated by, for example, MIMO channel metric measurement module 134in FIG. 1:det(H^(H)H)

Maximum capacity scheduling is also maximum CQI scheduling.

For scheduling based on maximum orthogonality for several MIMO channels,

$H_{1} = \begin{bmatrix}h_{11} & h_{12} \\h_{21} & h_{22}\end{bmatrix}$ $H_{2} = \begin{bmatrix}h_{11} & h_{12} \\h_{21} & h_{22}\end{bmatrix}$the following metric will be calculated by, for example, MIMO channelmetric measurement module 134 in FIG. 1:

$\max\left\{ {\sum\limits_{i = 1}^{2}\frac{\det\left( {H_{i}^{H}H_{i}} \right)}{{trace}\left( {H_{i}^{H}H_{i}} \right)}} \right\}$

For scheduling based on orthogonality capacity for several MIMOchannels, the following metric will be calculated by, for example, MIMOchannel metric measurement module 134 in FIG. 1:

$\max\left\{ {\sum\limits_{i = 1}^{2}{\det\left( {H_{i}^{H}H_{i}} \right)}} \right\}$

For scheduling based on a combined conditional number for several MIMOchannels, the following metric will be calculated by, for example, MIMOchannel metric measurement module 134 in FIG. 1:

$\min\left\{ {\sum\limits_{i = 1}^{2}\rho_{i}} \right\}$

For scheduling based on a SNR weighted maximum orthogonality scheme forseveral MIMO channels, the following metric will be calculated by, forexample, MIMO channel metric measurement module 134 in FIG. 1:

$\max\left\{ {\sum\limits_{i = 1}^{2}{{SNR}_{i}\frac{\det\left( {H_{i}^{H}H_{i}} \right)}{{trace}\left( {H_{i}^{H}H_{i}} \right)}}} \right\}$

For scheduling based on a SNR weighted capacity scheme for several MIMOchannels, the following metric will be calculated by, for example, MIMOchannel metric measurement module 134 in FIG. 1:

$\max\left\{ {\sum\limits_{i = 1}^{2}{{SNR}_{i}{\det\left( {H_{i}^{H}H_{i}} \right)}}} \right\}$

For scheduling based on a SNR weighted combined conditional numberscheme for several MIMO channels, the following metric will becalculated by, for example, MIMO channel metric measurement module 134in FIG. 1:

$\min\left\{ {\sum\limits_{i = 1}^{2}{{SNR}_{i}\rho_{i}}} \right\}$

FIG. 3A illustrates a table setting out one representative example of alookup table for selecting coding modulation and MIMO modes based CQIwhich can be used in accordance with one embodiment of the invention.The table in FIG. 3B provides a key for the table in FIG. 3A.

In the table of FIG. 3A, Row 1 lists possible CQIs, which in this casewould be from 1 to 10. Each CQI has an associated FEC code, anassociated modulation, and an associated MIMO mode, in this case an STCcode. In the particular example, illustrated, there are five availableFEC codes, three available modulations, and two available STC codes.

The key shown in FIG. 3B indicates the code rates 1/5, 1/3, 1/2, 2/3,4/5 associated with the five available FEC codes, indicates themodulation constellations 4-QAM, 16-QAM 64-QAM associated with the threemodulations, and indicates the STC modes STTD and BLAST associated withthe two available STC modes.

FIG. 3A shows how CQI, together with STC code rate, determines theproper code and modulation set. The table in FIG. 3A is of course justone possible example of a lookup table. The particular MIMO modes, FECcodes, and modulations supported will vary on an implementation specificbasis. The invention is in no way limited thereto. Extension of theprinciples of the present invention to others possible lookup tableswill be apparent to those skilled in the art. For example, other formsof lookup tables could be employed which include other standardmodulation schemes such as Phase-shift keying (PSK), and other forms ofMIMO modes such as beamforming.

FIGS. 4A and 4B illustrate a lookup table in which the CQI and MIMOI arefed back in separate feedback components. In this case, the MIMO mode isdetermined by both CQI and MIMOI.

FIG. 4A sets out one example for coding modulation based on CQI. Row 1lists possible CQIs which may be calculated by CQI metric measurementmodule 136, from 1 to 10. Row 2 lists Forward Error Correction Codes(FEC), which in this case are from 1 to 5. Row 3 lists three possibleforms of modulation which can be implemented in accordance with thislookup table, namely 4-QAM, 16-QAM, and 64-QAM. Reference may be had tothe table in FIG. 3B which provides a key for the table in FIG. 4A.

FIG. 4B is a lookup table for MIMO modes based on a MIMO indicator(MIMOI). MIMO indicator may be calculated by MIMO channel metricmeasurement module 134 shown in FIG. 1. In this case, Row 1 listspossible MIMO indicators from 1 to 10 which are selected based on theresult of the channel metric calculations. Row 2 lists possible MIMOformats (which could represent, e.g. BLAST, STTD, beamforming, spatialmultiplexing, etc.) from 1 to 5.

FIGS. 4A and 4B show that the higher the CQI/MIMOI, the higher is themodulation order and FEC coding rates. There are two situations where acomposite metric of both CQI and MIMOI may be used: the first is MIMOmode selection, because when MIMOI is low, SM may not be used even witha large CQI. FIG. 8 shows that at low MIMOI, in the high CQI region, theuse of STTD or SM as a MIMO mode depends on MIMOI. However, in the sameCQI region, when MIMOI is larger, SM is selected. Another situation ismodulation and code set selection in SM, because in SM, MIMOI indicatesinter-layer interference, and hence indicates the performance of thechannel. Given the same CQI, modulation and code set will depends on theMIMO indicator. In other words, modulation and code set will bedetermined by two parameters: CQI and MIMOI.

As with the table in FIG. 3A, the tables in FIGS. 4A and 4B are merelyillustrative examples of lookup tables which could be used in accordancewith the invention. Persons skilled in the art will appreciate thatother combinations of parameters can be employed.

FIG. 5 is a diagram illustrating another example of a lookup for MIMOmodes based on a MIMO indicator and a CQI. As noted above, a MIMOI maybe ascertained by calculations performed by MIMO channel metricmeasurement module 134 shown in FIG. 1 and CQI may be calculated by CQImetric measurement module 136 also shown in FIG. 1.

In FIG. 5, three sections are shown, a first section labelled “STTD”, asecond section labelled “spatial multiplexing (SM), and a third sectionlabelled “STTD/SM”. According to the lookup of FIG. 5, for low CQIs(i.e. low channel quality), regardless of the MIMO indicator, STTDshould be chosen as the MIMO format. For high MIMO indicators and highCQIs, the lookup of FIG. 5 indicates that spatial multiplexing should bechosen as the MIMO format. For a low MIMO indicator and high CQI, eitherof STTD and spatial multiplexing can be selected as the MIMO format.

For the example of FIG. 5, a CQICH (channel quality indicator channel)can be used to feedback coding/modulation information and/or selection,and a single bit can be used to flag the MIMO mode. One of ‘0’ or ‘1’can be used for STTD, and the other of ‘0’ or ‘1’ can be for SM.

FIG. 5 is of course just one possible example of a lookup in which theinvention may be implemented. The invention is in no way limitedthereto. Extension of the principles of the present invention to otherlookup diagrams will be apparent to those skilled in the art.

FIG. 6 is a plot of BLER (Block Error Rate) versus SNR (Signal-to-NoiseRatio) for an embodiment of the invention. FIG. 6 shows the impact onmaximum capacity scheduling. FIG. 7 is a plot of BLER (Block Error Rate)versus SNR (Signal-to-Noise Ratio) for another embodiment of theinvention. FIG. 7 shows the impact on maximum capacity scheduling. BothFIGS. 6 and 7 shows two sets of curves, namely, “without scheduling” and“with scheduling”. More particularly, without scheduling” means “randomscheduling”, and “with scheduling” means “orthogonality basedscheduling”.

FIG. 8A is a plot of Cumulative Distribution Function (CDF) versuscondition number of H^(H)H for an embodiment of the invention. H^(H)Hmeans Hermitian transform, i.e. conjugate transposition. FIG. 8Apresents the impact on minimum conditional number scheduling. FIG. 8Ashows the CDF of the composite metric when random pairing is used (thesolid line) and when the orthogonality based pairing is used.

FIG. 8B is a plot of CDH versus (HHH) for another embodiment of theinvention. FIG. 8B presents the impact on minimum conditional numberscheduling.

Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practised otherwise than as specifically described herein.

1. A method of operating a multiple-input-multiple-output (MIMO)transmitter, the method comprising: transmitting a first signal to atleast one receiver; receiving from the at least one receiver a compositemetric based on information pertaining to a MIMO channel metric andinformation pertaining to a channel quality indicator (CQI)corresponding to the transmitted signal, the composite metric specifyinga modulation and coding scheme and a MIMO mode; and transmitting asecond signal to the at least one receiver using a MIMO mode selected inaccordance with the composite metric and a modulation and coding schemeselected in accordance with the composite metric.
 2. The method of claim1, wherein the composite metric serves as a basis for selecting betweenat least two MIMO modes selected from a group consisting of BLAST, STTD,beamforming, spatial multiplexing, and a space-time mapping scheme. 3.The method of claim 1, wherein the MIMO transmitter is a MIMO orthogonalfrequency division multiplexing (MIMO-OFDM) transmitter.
 4. The methodof claim 1, further comprising receiving, from the at least onereceiver, MIMO capability information indicating MIMO capability of thecorresponding receiver.
 5. The method of claim 1, further comprisingselecting and scheduling at least one transmit antenna at the MIMOtransmitter for data transmission to the at least one receiver.
 6. Themethod of claim 5, wherein selecting and scheduling at least onetransmit antenna at the MIMO transmitter for data transmission to the atleast one receiver is based on the composite metric.
 7. A method ofoperating a multiple-input-multiple-output (MIMO) receiver, the methodcomprising: receiving a signal at the receiver from at least onetransmitter; generating a MIMO channel metric and a channel qualityindicator (CQI) based on the received signal; determining a compositemetric based on the MIMO channel metric and CQI, the composite metricspecifying a modulation and coding scheme and a MIMO mode; andtransmitting the composite metric to the at least one transmitter. 8.The method of claim 7, wherein the composite metric serves as a basisfor selecting between at least two MIMO modes selected from a groupconsisting of BLAST, STTD, beamforming, spatial multiplexing, and aspace-time mapping scheme.
 9. The method of claim 7, wherein determininga composite metric comprises determining a composite metric whichspecifies for the MIMO mode: a space time block code when the CQIindicates a low channel quality regardless of the MIMO channel metric;spatial multiplexing when the CQI indicates a high channel quality andthe MIMO channel metric is high; and one of a space time code block anda spatial multiplex mode when the CQI indicates a high channel qualityand the MIMO channel metric is low.
 10. The method of claim 7, furthercomprising transmitting to the at least one transmitter MIMO capabilityinformation indicating MIMO capability of the receiver.
 11. Amultiple-input-multiple-output (MIMO) transmitter system, comprising: atransmitter operating to transmit a first signal to at least onereceiver; a receiver operating to receive, from the at least onereceiver, a composite metric based on information corresponding to aMIMO channel metric and information corresponding to a channel qualityindicator (CQI) relating to the transmitted signal, the composite metricspecifying a modulation and coding scheme and a MIMO mode; thetransmitter further operating to transmit a second signal to the atleast one receiver using a MIMO mode selected in accordance with thecomposite metric and a modulation and coding scheme selected inaccordance with the composite metric.
 12. The transmitter system ofclaim 11, wherein the composite metric serves as a basis for selectingbetween at least two MIMO modes selected from a group consisting ofBLAST, STTD, beamforming, spatial multiplexing, and a space-time mappingscheme.
 13. The transmitter system of claim 11, wherein the MIMOtransmitter is a MIMO orthogonal frequency division multiplexing(MIMO-OFDM) transmitter.
 14. The transmitter system of claim 11, whereinthe receiver operates to receive from the at least one receiver MIMOcapability information indicating MIMO capability of the receiver. 15.The transmitter system of claim 11, wherein the transmitter furtheroperates to select and schedule at least one transmit antenna at theMIMO transmitter for data transmission to the at least one receiver. 16.The transmitter system of claim 15, wherein the transmitter operates toselect and schedule at least one transmit antenna at the MIMOtransmitter for data transmission to the at least one receiver based onthe composite metric.
 17. A multiple-input-multiple-output (MIMO)receiver system, comprising: a receiver operating to receive a signalfrom at least one transmitter; a generator operating to: generate a MIMOchannel metric and a channel quality indicator (CQI) based on thereceived signal; and determine a composite metric based on the MIMOchannel metric and CQI, the composite metric specifying a modulation andcoding scheme and a MIMO mode; and a transmitter operating to transmitthe composite metric to the at least one transmitter.
 18. The receiversystem of claim 17, wherein the composite metric serves as a basis forselecting between at least two MIMO modes selected from a groupconsisting of BLAST, STTD, beamforming, spatial multiplexing, and aspace-time mapping scheme.
 19. The receiver system of claim 17, whereinthe generator operates to determine a composite metric by determining acomposite metric which specifies for the MIMO mode: a space time blockcode when the CQI indicates a low channel quality regardless of the MIMOchannel metric; spatial multiplexing when the CQI indicates a highchannel quality and the MIMO channel metric is high; and one of a spacetime code block and a spatial multiplex mode when the CQI indicates ahigh channel quality and the MIMO channel metric is low.
 20. Thereceiver system of claim 17, wherein the transmitter further operates totransmit to the at least one transmitter MIMO capability informationindicating MIMO capability of the receiver.